1. Field of the Invention
The present invention relates to an optical fiber laser for emitting excitation light incident on a side of an optical fiber having a gain core which is used as a gain medium and relates to a method of excitation by the optical fiber laser. In particular, the present invention relates to an optical fiber laser for efficiently focusing excitation light in the gain core and relates to a method of excitation by using the optical fiber laser.
2. Description of the Related Art
Development of a high-output, inexpensive light source intended to be used for laser processing or in medical applications is demanded. To meet this demand, much attention is paid to optical fiber lasers and optical amplifiers such as optical fiber amplifiers since they are highly efficient and can obtain single-mode laser light with ease.
Double-clad fibers (DCFs) are generally used, which have a core to which a rare earth element is added as a gain medium. Exemplary excitation methods of the optical fiber laser in which a laser bar emits excitation light to the double-clad fiber include: a method in which a combiner is used so as to converge incident excitation light from a plurality of light sources; another method in which a fiber bundle is fused and excitation light is incident on segments of the fiber bundle; another method in which a micro lens is used to cause excitation on an end surface; and another method in which a tapered light guide is used to enter excitation light.
In the above methods, however, it becomes necessary to provide a combiner, a fiber bundle, a micro lens, a light guide, and other optical devices between the laser bar and double-glad fiber. The use of these intervening optical devices is problematic in that they are expensive and light emitted from the laser bar is lost by the optical devices before the light reaches the double-clad fiber.
Thereto, Tanaka et al. disclose in JP-A-2001-36170 (U.S. Pat. No. 6,533,883) that a laser in which: a laser fiber wound in a spiral form is integrated into one piece by heating thermoplastic resin; a plurality of lens ducts with excitation light laser diodes are attached to the outer circumferential side of the resulting optical conductor; and laser light beams amplified by emitting excitation light are obtained. However, since the laser disclosed in JP-A-2001-36170 needs many lens ducts with excitation light laser diodes, it cannot be said that the amplification efficiency of the laser is high.